The intensity of light from a continuously emitting laser source operating at 638 nm wavelength is modulated at 1 GHz . The modulation is done by momentarily cutting the intensity off with a frequency of 1 GHz . What is the farthest distance apart two detectors can be placed in the line of the laser light so that they can see the portions of the same pulse simultaneously ? ( consider the speed of light in air `3 xx 10^8 m//s` )

Options

A. `30 mu` m
B. `30 cm`
C. `3 m`
D. `30 m`

Answer

B

Explanation

Time period between two flashes `= (1)/(f)`
Distance travelled by laser in this interval
`= (c )/(F ) = ( 3 xx 10^8 )/( 10^9) = 0.3 m = 30 cm`
So this is the maximum distance between two detectors, so that they can see the same pulse simultaneously